"But more voted for her than voted for any other person. (or, at least, voted for their local Conservative candidate)"

I think that says it all really. People also vote based on party or local candidate and not necessarily only on who is the party leader. Plus of course for at least part of her tenure she didn't have much opposition as Labour was in disarray. I seem to remember Blair was pretty unpopular by the time of his last election victory yet the party still won.